You need a high loyalty level on their Diplomatic boxes. But in the case of Sweden there is an issue not working properly.
Sweden needs Denmark surrender first and this is not happening against the AI because she never takes all the cities needed to this (Hamburg, Kiel, Aalborg, Lubeck, etc...)
If the AI is playing the PRO we have another issue as she doesn't play the Decisions to raise the Loyalty in the Diplomatic boxes.
We are testing these events in order to release a beta patch ASAP.